Traveling-wave tube



June 14, .11960 H. R. JOHNSON EI'AL 2, 41,

TRAVELING-WAVE TUBE Filed April 1, 1957 2 Sheets-Sheet 1 Fig.1.

Fig. 2.

' Arm-Mm. zwmua Horace R. Johnson,

Rolf D.Weglein,

June 14, 1960 H. R. JOHNSON ETAL 2,941,113

TRAVELING-WAVE TUBE 2 Sheets-Sheet 2 Filed April 1, 1957 Fig. 3.

Fae/0a: M005 4M5 ZM/ o w w 5 zw uu mu 4 556 5 a V o m" h I) Olww 9 e v w m F A 0'' 0 R y A 4 I F United States Patent TRAVELING-WAVE TUBE Horace R. Johnson and Rolf D. Weglein, Los Angeles, Calif., assignors to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed Apr. 1, 1957, Ser. No. 649,836

2 Claims. (Cl. 315-35) The present invention relates in'general to travelingwave tubes and more specifically to means for precluding spurious backward-wave oscillations in a traveling-wave tube.

Conventional traveling-wave tubes have found Widespread use as microwave amplifiers because of their ability to provide useful gain over a broad bandwidth, especially when utilized as forward-wave amplifiers. Traveling-wave tubes generally comprise a slow-wave structure for propagating electromagnetic signal waves at a velocity considerably less than the velocity of light. An electron stream is then projected along the slow-wave structure in electromagnetic energy exchange relation with the traveling wave in a manner thus effectively to push or amplify the traveling waves.

When a traveling-wave tube is utilized as a backward wave amplifier or backward-wave oscillator, the tube is quite dispersive and is therefore a narrow-band device. That is, for a given velocity of electrons in the stream only a very narrow range of frequencies may be amplified in a backward wave, that is, a wave traveling in the direction opposite to that of the electron stream. In order that backward-wave oscillations may occur at a given irequency, a certain minimum starting current at a certain electron velocity 'is required.

It may be seen that when a traveling-Wave tube is being utilized as a forward-wave amplifier, with the electrons in the stream traveling at a certain velocity, backward-wave oscillations may occur at the frequency determined by the dispersion characteristic for the tube as a backwardwave oscillator, provided there is a sufiicient current of electrons at that certain velocity for the start-oscillation condition. Such backward-wave oscillations would be undesirable in a forward-wave amplifier.

Formerly, this backward-wave oscillation has been avoided in forward-wave amplifiers through the use of smaller helices or other circuits of low backward-wave impedance. Another method is disclosed and claimed in the copending application by Horace R. Johnson, et al., Serial No. 401,303, filed December 30, 1953, now Patent No. 2,809,321, dated October 8, 1957, entitled Traveling- Wave Tube, and assigned to the assignee of the present application, which comprises using appropriate stop-bands for the purpose of rejecting the spurious backward-wave oscillations. The first method, viz., that of using a smaller helix, is limited to low-power tubes, since a smaller helix means a smaller inner passage or opening for the stream and hence limits the amount of total current in the electron beam. Also, special circuits can often not be utilized since they are usually quite dispersive and hence narrow-band devices. Dispersion is therefore also a problem with the latter method of utilizing stop-band circuits.

It is therefore an object of thepresent invention to provide a high-power traveling-wave tube which does not suifer the disadvtnages of spurious backward-wave oscillations.

his a further object to provide a backward-wave oscillator in which spurious modes of oscillation are prevented.

It is another object to provide such a traveling-wave tube without modifying the cold slow-wave structure, for example, its dispersion.

Briefly, these and other objects are achieved by provid-' ing a multi-velocity electron stream in a manner such that either fundamental mode backward-wave oscillation or highpower amplification may be produced, while at the same time having not enough stream electrons at a particular velocity to constitute the starting curent required for spurious backward-wave oscillation frequencies. This velocity spread is produced by providing a steady electric field transverse to and across the electron beam. This may be done, for example, in a conventional helix type hollow beam traveling-wave tube by supporting a conducting rod along the axis of the traveling-wave tube inside of the hollow electron beam. This inner conductor is then maintained at a direct-current potential which is difierent from that of the surrounding helix. Various other means associated with other types of traveling-wave tubes for achieving a velocity distribution with such a transverse field are illustrated, discussed and explained in the following detailed description and drawings in which:

Fig. 1 is a sectional view of a traveling-wave tube constructed in accordance with the present invention;

Fig. 2 is a graph of the dispersion characteristic of the traveling-wave tube plotting velocity of electrons on the ordinate with microwave frequency on the abscissa; also shown is the beam current distribution versus velocity;

Fig. 3 is a sectional view of an alternative embodiment of the present invention; and

Fig. 4 is a plot of starting current on the ordinate versus velocity spread of the electron stream on the abscissa.

Referring with more particularity to the drawings for purposes of description of structure and operation of the present invention, Fig. 1 illustrates an embodiment in which the transverse electric field across the electron stream is achieved in a radially symmetrical configuration. To this end a dielectric rod 10 with a conductive coating 12 is supported along the axis of a traveling-wave tube 14. The electrical potential of conductive coating 12 is maintained ditierently from and, in the case of a backwardwave device, more positive than that of a conductive helical slow-wave structure 16; in a forward-wave device the sense of the transverse field may be reversed. A hollow cylindrical electron beam produced by electron gun 18 is caused to pass through the cylindrical annular space between rod 10 and slow-wave structure 16, across which is maintained a steady direct-current potential. The electron stream is collected by a collector electrode 20 which is appropriately designed to effectively dissipate the kinetic energy of the stream electrons.

Electron gun 18 comprises an annular emitting cathode 22, a beam-forming electrode 24, a first accelerating electrode 26 and a second accelerating electrode 28. Each of the electrodes 26 and 28 have an appropriate center portion coplanarly supported with the outer electrode, as by grid wires or by a common supporting dielectric rod not shown, as is conventional in the art. Dielectric rod 10 is supported by and between the center portion of accelerating electrode 28 and collector electrode 20. A coaxial line 3i) forms an input line if the tube is being operated as a forward-wave device, or may be used as an output line if operated as a backward-wave device, and is connected to the left-hand end of slow-wave structure 16, as may be coaxial line 32 to the right-hand end of slow Wave structure 16 which is an output line if the tube is aforward-wave device, or an input line if a backwardwave amplifier.

An axial magnetic'field for focusing and constraining stream electrons is provided by an outer solenoid 34 energized by a battery 35. Appropriate taps on a source 7 area frequency of "backward-Wave oscillations,

outside thereof. Y

t a em of potential 35 are electrically connected to each of the various electrodes to provide operating voltages thereon. One such tap is labeled '38 and its position determines therelative potentiahdesignated V of conductive coating 12 to which it is'connected. 'Ihisin turn'determ nes the potential gradient between the conductive coating'and slcpw-wavestructure 16, the potential'of which in this example is maintained'at ground, designatedfas' V through ahisolatihg resistor 49; The potential of collector elec- @6520 is maintained somewhat positive with respect to that of the conductive coating '12 or slow-wave structure "16 iniorder to minimize the deleterious efiects of secondarily-emitted electrons therefrom. V 7

graphvo f Fi 2 is provided for the purpose of depic'ting the" dispersive characteristic of backward-wave oscillations associated with the traveling-wave tube of Fig. 1 Lwhen being operated as a forward-wave amplifier. Such backward-wave oscillations woud'be-deemed spurieas aaa; undesirable when the tube'is utilized as a forward wave amplifier. The graph shows curve 43 plotvelocity o'fstre'am electrons versus the frequency of backward-wave traveling waves, the amplification of which would be effected by the indicated velocity of stream electrons in the forward direction.

' The shaded area 42, limited by curve 41 independently represents the distribution of beam current electrons versus velocity which extend throughout the range of V to V the potentials on conductive coating 12 and slowwave "structure 15, respectively. The distance on the distributiongraph,indicated by the bracket 44, represents the starting icurrent for backward-waveoscillations to "be suppo ted;

"The diagram of Fig. 2 points out that while'forwardwave amplification'may be'achieved by beam current electrons whose velocities are within the rangetroin Y Y2, backward-wave oscillation is precluded because nowhere in this'distributionai'e there suflicient electrons t'o constitute the required'starting current for the'associ- Thus, there is provideda means for-high-power amplification withoutbackward-Waveoscillation because a large mag:

nitride of integrated stream current couldlbe'used wjithont havingsufiicient electrons to constitutethe starting current required {for backwardfwave oscillations V v Fig. '3 illustrates "an "embodiment of "the present inven- It has been found, when a backward-wave oscillator operating in its fundamental mode as represented by a position on curve 78 is provided with a transverse, time constant or steady electric field across the electron beam, that the fundamental m'o'de'o'f oscillation is affected in a quite diflerenttway' than aretlie higherorder, spurious modes, the first and most troublesome of which is repres'ent ed by the curve 80;"

As may be seen from the graph of .Fig. 4 which represents the operation of abackward-wave oscillator c'onstructed substantiallyi'n accordance with Fig.' 1 where V is the voltage on.the innerconducting rod and V is the voltage on the helixfwhen V ispositive with'respect to V such that the tube is operating somewhat to the right of 0 of the abscissa, for example, near the point .1,

the starting c tIent; tor the fundamental, mode is. inc ieiised relatively little from its value given byrc rv .78 at he point .Q? where V jequa1s V 011 the'otherhand, it may be seen from curve 80 that the starting current for-the p i u 'mc e p qe h sinfi ity 7 I t a Typical empirical data follow below for a, b ackwardwave oscillator constructed in accordance with Fig. '1 and operating near the point .lf to the right of in F 4 Where t nn co duct ro is qnstmcted of ceramic coated with silver and ismaintainedat a potential P i e 'g es ec to the en r conducting hel xh electric el b twe n the in r rod nd the-outer. h l xis apr qxim l Q 1t per n h; V1 isrntthe order of ZOQO volts positive with respecttovthe cathode; the beam current is in the range of :10 m; 1 2 inilliamperes;

, the-frequency of operation isin theregion of 3490 1913500 me cy l s; he "a ria ns r ni ag e ic field is 10.00 ems; nd the starting current tor; the spur ous anode is. infinity. r

. he i -s nslli tire a i readi y rl cedsufl ci nuvf radially difi l f q sheli Suc a z Q to interim W wa ba thexealen n m mwaves th W l er wa eatra d E l ?lli Q.. O.t 'll i ch? an re es ea tfi ld sir .r e a l seen :by visualizing the 5 resultant f gradient across which tib'rfiiihich may be considered the conjugate of the *at- V rangenient shown in Fig. l in that as'low-.wavetstructiire -'16" lies along the axis of a traveljng wave tube 14, while a concentric tubular conductor 12' lies outsidethereof with 'anianh'ular cylindrical space inbetween' through which maybe projected a hollow beam of electrons generated by an electron "gun 18. The outer conductor 12' may be any n'onrnagnetic conductor or coating adaptable to achieve the function 'of providing a steady'direc't-CUI- rent potential across the i hollow electron beam. "Slowwave structure 16 is of a character well kn'own in the art and is desc'ribe'dtor examp'le in'Patent Nd. 2,645,737, entitledi Traveling wave Tube, issued'to L. M; Field on luly l l, 1-953. Condu'ctive 'finsor'dises46 are substantiallyievenly-spaced along" axial central conductor '48 which is functionally an extension of "theinne'r' conductor th s etm st sa read- J h ec an adiall n e t theme; nqs ve s nsler: Wil13li Q 1Ti1YZh3YQ meresi er a Thence h travel n fa e ha tho e in 3 re e a ve i ipu 0 t esret ie tr nsve s omp n n nte q tys vento the ekemasbuniis.

l isle e n -ma t st which c lly aids ilJaQQIl of'coaxial feed lines 30' 32; "The outer conductor 7 or these teed' lines issiinilarly'extended' into travelingwave tube 14 withfiared endsstl which aid in achieving suitable impedance match between coaxial 'line' an'jd Eleetro-magnetically transparent slow-wave 'structure. windows' '52 permit the desired pressureidifierential be tween theevacuated interior oftraveling-wavetube -14 and Rent might be; ts} en-ample, gatmospheric pressure Reterrin'g nowto the ,igraphjot Fig. 4, ,thereis shown ap'lot' of "backward-wave oscillations 'startin'gcurrent. in arbitrary units versus velocity spread in the electron beam g'gfl abscissa expressed as-aratio which ispr bbr- @isnal taaprss etarmiasd by repulsive Sprcecherge-sprdfldill effects.

fihere have thus been disclosed travelingawave; tubes which, when ,operatedas iorward-wave amplifiers or :as

7 backward wave amplifiersior oscillators, may utilize ;a

' the case "at Fig; l the 3 innerrconductive rod is 3 in-;the

atima??? that it o in rie e vwithwaves i sas m -e ena henn nhehx: I A st rt e adv m s atthe misrzeend q i ourney; ew i dt hefqw iaso e t 9n esme n t tsen mode of oscillation comprising: a conductive helix for the propagation of electromagnetic wave energy at a velocity substantially less than the velocity of light; an electron gun for producing a hollow stream of electrons projected contiguously within and in energy exchange relationship with said helix; output means coupled to the electron gun end of said helix; an elongated dielectric rod coated substantially over its length with a resistive layer and being disposed within said hollow electron stream coincident with the axis of said traveling-Wave tube whereby said hollow electron stream flows between said elongated rod and said helix; means for providing a magnetic field substantially parallel to said axis for focusing said stream; and means for maintaining the resistive layer on said dielectric rod at a steady direct current potential which is positive with respect to said helix whereby a transverse direct current electric field is maintained between said helix and said rod across said hollow electron stream in a manner such that said hollow electron stream sufiers a velocity spread in a manner to preclude spurious backward-wave oscillations in said backward-wave oscillator traveling-wave tube, said transverse direct current electric field being -in the range of magnitude to not appreciably increase the magnitude of oscillation starting current for said fundamental mode.

2. A backward-wave oscillator traveling-wave tube for generating signals in a predetermined, fundamental mode of oscillation comprising: an electron gun; a helix for propagating high frequency electromagnetic energy at an axial velocity substantially less than that of light and having output means at its electron gun end and being maintained at a direct current potential of V a coaxial, substantially coextensive dielectric rod having a resistive, nonpropagating layer on the outer surface thereof and disposed within and radially spaced syrn metrically from said helix and maintained at a direct current potential of V where V is positive with respect to V to provide a radial potential gradient and an electric field of the order of 1000 volts per inch between said heiix and said rod; means for providing a hollow electron stream projected coaxially along the length of the tube, electrons of said beam having a distribution of radial displacement between said helix and said rod in the direction of said gradient, those electrons passing nearer to said rod being in a region of higher potential and traveling at a higher velocity than those electrons nearer to said helix thereby providing a velocity spread in said electron stream in accordance with said voltage gradient, said velocity spread being of a magnitude to not appreciably increase the magnitude of required starting current for said fundamental mode but to make prohibitively large the starting current for a spurious mode of backward-wave oscillation; and means for providing a strong axial magnetic field for constraining said hollow electron stream to a substantially rectilinear flow along the length of said helix.

References Cited in the file of this patent UNITED STATES PATENTS 2,610,308 Touraton et al. Sept. 9, 1952 2,643,353 Dewey June 23, 1953 2,730,649 Dewey Jan. 10, ;1956 2,742,588 Hollenberg Apr. 17, 1956 2,761,088 Warnecke et al. Aug. 28, 1956 2,812,467 Kompfner Nov. 5, 1957 2,843,792 Pierce July 15, 1958 2,843,793 Ashkin July 15, 1958 

